1. Field of Invention
This invention relates to pressure sensitive adhesives, which can be applied to a substrate as a melt, and more particularly to such adhesives based on copolymers of ethylene and n-butyl acrylate and optionally acrylic acid or methacrylic acid and ionomers of such acid copolymers.
2. Description of Related Art
Hot melt pressure sensitive adhesives (HMPSA) are one of the most widely used types of adhesives used today. They are typically coated as a melt onto a substrate, and the resulting adhesive coating operates as a pressure sensitive adhesive at ambient temperature. Substrates may be paper, metal, foil, plastics etc., and when coated with HMPSA may be useful as tapes, labels, floor covering, bookbinding and hot melt sealants. HMPSA may also be used for adhesion of metals, plastics, wood or other materials.
Block copolymers of styrene-isoprene have been used in HMPSA, but these adhesives exhibit deficient thermal, oxidative, and UV stability and oil resistance arising from the properties of the block copolymers.
Ethylene vinyl acetate (EVA) copolymers have been used as hot melt adhesives (HMA). These copolymers have had little commercial acceptance in HMPSA adhesive composition however, for several reasons. First, the EVA tends to bleed into and through permeable substrates, particularly at slightly elevated temperatures. Second, the EVA has known shortcomings with respect to thermal stability. U.S. Pat. No. 4,338,227 to Ballard discloses the use of compositions based on crosslinked EVA copolymers as HMPSA to avoid the bleeding problem and to reduce the adhesive cold flow at 50.degree.-60.degree. C. Even so, crosslinked EVA based HMPSA compositions have not been commercially successful to date. Thus, the most popular copolymer used in HMA has not found utility in HMPSA.
The use of ethylene/n-butyl acrylate copolymers having a melt index of at least 50 in HMA compositions is disclosed in U.S. Pat. Nos. 4,816,306 and 4,874,804 to Brady et al. These patents disclose the use of tackifiers and a high melting synthetic wax in the HMA. There is no disclosure in these patents of the compositions being useful as HMPSA and indeed, the presence of the wax would prevent such use.
U.S. Pat. No. 3,849,353 discloses hot melt compositions useful as HMPSA which contain the combination of (a) copolymers of ethylene and vinyl acetate and/or alkyl acrylate; (b) atactic polypropylene; and (c) vulcanized rubber. Of the alkyl acrylate copolymers possible, ethylene/butyl acrylate is one of the preferred. Components (a) and (c) are disclosed to provide cohesive properties to the adhesive and component (b) is disclosed to provide compatibility with tackifiers. Component (b) by itself is somewhat tacky at ambient temperature. Component (a) is present in amounts up to 50% based on the weight of (a), (b), and (c) and components (b) and (c) make up the remainder of the composition. Tackifier may be added to this composition in an amount up to 50% of the weight of the composition.
There is a commercial need for a HMPSA having good thermal, oxidative and UV stability which does not bleed or lose its adhesive performance at somewhat elevated temperatures.